1. Field of the Invention
The present invention relates generally to a differential amplifier. More specifically, the invention relates to a differential amplifier adapted to be integrated in C-MOS monolithic IC.
2. Description of the Related Art
An example of a differential amplifier of C-MOS structure having a conventional common-mode feedback circuit has been disclosed in Japanese Patent Publication No. 2-44404. The circuit disclosed in the foregoing publication will be discussed with reference to FIG. 3.
Differential inputs 101 and 102 are applied to respective gate electrodes of NMOS transistors 1 and 2 as differential transistors having sources connected in common. A constant current NMOS transistor 5 is provided for supplying a constant current to a common junction of the sources of the NMOS transistors 1 and 2. A bias is applied to the gate of the NMOS transistor 5. PMOS transistors 3 and 4 are active loads of the transistors 1 and 2, respectively.
With these transistors 1 to 5, an input stage differential amplifier 200 is formed. A differential amplifier output of this input stage differential amplifier 200 is obtained from respective drains of the transistors 1 and 2.
The input stage differential amplifier output serves as a differential input of an output stage differential amplifier 201. The input stage differential amplifier output is thus applied to respective gate inputs of a pair of PMOS transistors 6,7 having source connected in common. As active loads for a pair of transistors 6 and 7, NMOS transistors 8 and 9 are provided. A bias 106 is applied to the gates of the transistors 8 and 9.
The amplifier outputs 103 and 104 of the output stage differential amplifier 201 are output through respective drains of the transistors 6 and 7. The signal at the differential inputs 101 and 102 is amplified to be obtained through the differential outputs 103 and 104.
Next, discussion will be given for the common-mode feedback circuit. PMOS transistors 10, 11 which take the differential outputs (drain outputs of the transistors 1 and 2) as gate inputs and convert the differential output voltages into current, are provided. The drain outputs of the transistors 10 and 11 are connected in common for current composition. The composite current corresponds to a common mode output voltage of the input stage differential amplifier.
The composite current of a pair of common-mode current generating transistors 10 and 11 is an input mirror current of a current mirror circuit 202. The current mirror circuit 202 comprises NMOS transistors 12 and 13. The composite current is supplied to the drain of the transistor 12. An output mirror current is generated at the drain of the transistor 13.
The output mirror current is supplied to the drain of a PMOS transistor 17. Here, since another current mirror circuit is formed by the transistor 17 and the active load transistors 3 and 4, the common-mode output voltage appearing at the respective drain of the transistors 3 and 4 as the output mirror current are fed back.
By providing common-mode feedback, variation of a common-mode output of the differential amplifier can be suppressed.
It should be noted that transistors 20 and 22 and capacitors 21 and 23 form a frequency compensation circuit, and the transistors 20 and 22 are adapted to serve as resistor employing depression type MOS.
Such conventional common-mode feedback circuit has three paths, i.e. the voltage-current converting portion of the transistors 10 and 11, the current mirror circuit portion of the transistors 12 and 13 and the current mirror portion of the transistors 17, 3 and 4. Therefore, the common-mode feedback circuit is equivalent to two stage amplifier construction. Accordingly, the transfer function between the input and output of the common-mode feedback circuit becomes a transfer function having two poles to make common-mode feedback unstable and to case oscillation in the worst case.